This invention relates to the apparatus and methods suitable for storage vessel systems typically located at customer or user sites which receive liquid carbon dioxide (CO.sub.2) by truck or rail and then supply it to devices where it is expended so as to create a refrigeration effect, and especially useful when creating a low temperature effect of below 0.degree. F., and as low as -110.degree. F.; and such systems, while they may have other beneficial uses, are especially useful as ground support/filling apparatus and arrangement for trucks or rail cars using CO.sub.2 for cooling.
Liquid carbon dioxide has long been used both as a working (typically closed cycle) and as an expendable (typically open cycle) refrigerant for many low temperature applications because of a number of factors. Its non-toxicity, its desirably low range of refrigeration temperatures, its positive pressures at low temperatures and its lack of a residue (other than vapor) after having given up its refrigeration effect are among the more important. Dry ice (a solid), at atmospheric pressure, sublimes to a vapor at -110.degree. F. In recent years, CO.sub.2 's use as a working refrigerant has declined because of its high condensing pressure at ambient temperatures. However, CO.sub.2 's use as an expendable refrigerant has greatly increased. When used as an expendable refrigerant, liquid CO.sub.2 is generally stored at a customer site in an insulated pressure vessel under about 300 psig pressure and at a temperature of about 0.degree. F. and then used by the customer in a variety of manners. Most frequently the liquid CO.sub.2 is piped to a machine where it is expended. These machines are generally characterized within the CO.sub.2 industry as "dispensing devices" or "dispensing equipment." The liquid CO.sub.2 within the storage vessel is typically maintained at 0.degree. F. temperature and 300 psig pressure by means of a mechanical refrigeration system (freon, R-404A or similar), where the mechanical system's freon evaporator is a coil located within the storage vessel's ullage volume and the freon condenser is fan cooled by ambient air. By this means, CO.sub.2 vapor is able to be condensed to liquid inside the storage vessel, whenever the pressure in the storage vessel rises, for whatever reason, to an undesired level.
In many cooling applications, such as filling the dry ice bunker of a rail car or such, as shown in U.S. Pat. No. 5,660,057 issued to the present inventor, the liquid CO.sub.2 (supplied by a ground support/filling system) is then expanded inside the bunker to atmospheric pressure, where it partly turns to a solid, termed CO.sub.2 snow (a loose form of dry ice), with a substantial portion of the liquid CO.sub.2 flashing to vapor. As shown in the '057 patent, the use of liquid CO.sub.2 at a temperature below 0.degree. F. is desirable in such applications because the use of such colder liquid CO.sub.2 produces a larger percentage of solid CO.sub.2 and a smaller percentage of vapor CO.sub.2. Reductions in both CO.sub.2 usage of up to about 20% and in filling times are made possible by the use of such colder liquid CO.sub.2. In some applications, such as enroute truck cooling, as shown in U.S. Pat. Nos. 4,045,972; 5,267,443 and others, liquid CO.sub.2 is carried on board the truck in small tanks, and advantageously at pressures in the range of 110 psig, where its equilibrium temperature is about -50.degree. F.; and each truck's individual small tank must be frequently re-filled. Again, reductions in CO.sub.2 usage and much faster filling times would result from the use of CO.sub.2 colder than 0.degree. F. While truck and rail car cooling systems especially benefit from a low temperature liquid CO.sub.2 ground support/filling apparatus and systems (i.e. a non-mobile/liquid supply system), there are many other uses in diverse applications which would benefit from having colder CO.sub.2 available.
While cooling CO.sub.2 at a user's site may seem to be a straightforward mechanical refrigeration problem, a number of factors have prevented any wide use. U.S. Pat. No. 4,377,402 issued Mar. 22, 1983 shows a binary cascade (R-13/R-502) mechanical system that cools liquid CO.sub.2 as it flows between the storage vessel and the use point. Accordingly, the mechanical refrigeration system must be sized sufficiently large to match the highest instantaneous CO.sub.2 flow rate, which results in a very large mechanical system, especially burdensome since the CO.sub.2 may only be used a few hours per day. U.S. Pat. No. 3,660,985 issued May 9, 1972 to the present inventor represents a different approach where a small reservoir is filled with colder and depressurized liquid CO.sub.2 for intermittent use, but the reservoir having to be repressurized before each use, and again the mechanical refrigeration system must be sized large enough to match the highest instantaneous use rate of the CO.sub.2. U.S. Pat. No. 3,754,407 issued Aug. 28, 1973 to the present inventor, illustrates a system for filling remote reservoirs with very cold CO.sub.2 from a system that cools the CO.sub.2 as it flows from the storage vessel to the remote reservoirs, by means of a binary cascade system (CO.sub.2 /freon), with the freon system being that typically associated with the storage vessel. But again the size of the mechanical refrigeration system had to be large enough to match the highest instantaneous use rate of the CO.sub.2. U.S. Pat. No. 4,693,737 issued Sept. 15, 1987 to the present inventor illustrates a somewhat similar system for filling remote reservoirs, providing cooling by means of a small reservoir containing slush CO.sub.2 (a mixture of liquid and solid); maintained by a binary cascade (CO.sub.2 /- - -) system, with the upper stage being the type typically associated with the storage vessel. U.S. Pat. No. 4,695,302 issued Sept. 22, 1987 to the present inventor illustrates a somewhat similar system, except small tanks on board rail cars are being filled with liquid CO.sub.2.
U.S. Pat. No. 4,888,955 issued Dec. 26, 1989 to the present inventor, et al, represents an effort to both eliminate the separate vessels of the earlier solutions and to provide a refrigeration system sized for the average daily use rather than the instantaneous use. Using a vertically oriented storage vessel containing the standard freon type refrigeration unit's evaporator coil in the ullage volume, it maintained a normal storage vessel pressurization of approximately 300 psig, while cooling to near -50.degree. F. the lower portion (producing sub-cooling) in a single storage vessel by creation of a thermocline within the stored liquid CO.sub.2. It also used an unusual type binary cascade (CO.sub.2 /freon) system to cool liquid CO.sub.2 removed from the storage vessel to near -50.degree. F. (by direct heat exchange with the freon) and then return the cooled CO.sub.2 to the bottom of the storage vessel. The -50.degree. F. CO.sub.2 portion was liquid CO.sub.2 that had been sub-cooled by a freon unit. The CO.sub.2 vapor condensing machinery was the standard refrigeration unit typically associated with and part of the storage vessel and the CO.sub.2 portion of the binary cascade system was only used to provide 0.degree. F. liquid CO.sub.2 for sub-cooling of the freon after it had been condensed in an ambient heat exchanger. While this system overcame the instantaneous refrigeration sizing shortcomings of the earlier systems; its low temperature freon portion was such that the freon compressor was forced to operate with most difficult compression ratios (in excess of 15) if temperatures as low as -50.degree. F. were to be reached; without considering the temperature approach inefficiency of its -50.degree. F. freon to CO.sub.2 heat exchanger.
The nature of CO.sub.2, combined with the standard past practices of the industry and the existing plant production and distribution equipment, that is to almost a world-wide standard, all combined to present safety related problems encountered when filling storage vessels containing very low pressure/cold CO.sub.2. Most modern liquid CO.sub.2 production plants are in fact by-product plants, and being part of a much larger production complex are not readily changed, as are the large fleets of distribution equipment (cars, trucks, etc.). The temperature/pressure at which the liquid CO.sub.2 is produced from these plants is typically about -20.degree. F./225 psig, and heat gains to the liquid CO.sub.2 during subsequent distribution can raise the temperature/pressure to about 0.degree. F./300 psig, all thus resulting in a wide range of conditions at which liquid CO.sub.2 is delivered to the various storage vessels by the trucks or rail cars, designed for these temperatures and pressures. A safety problem has been encountered in the past when delivering CO.sub.2 to a storage vessel that contains low temperature-low pressure CO.sub.2. This problem occurs, if in transferring the liquid CO.sub.2, from the delivery truck or car, the pressure in the receiving vessel is so low, that the liquid CO.sub.2 still in the delivery truck or car becomes cooled by depressurization to below the safe operating temperature of the truck or car's material(s) of construction. Typically, liquid CO.sub.2 is pumped from the delivery vehicle into the storage vessel, and in the process, CO.sub.2 vapor from the storage vessel is forced back into the delivery vehicle through a separate return line. The entrance to this return vapor line is placed sufficiently below the top of the storage vessel to provide the ullage volume necessary for safe operation - so as to not liquid fill the storage vessel.
Another problem encountered in the past was that the change in phase and/or cooling occurring when depressurizing or cooling liquid CO.sub.2 can cause various impurities in the liquid CO.sub.2 (even if in very minute quantities) to agglomerate and then tend to separate or collect out of the liquid CO.sub.2 ; and unless removed or isolated, cause many problems in the storage vessel system or in the dispensing devices it serves. Such impurities can be in the form of non-condensables or condensables or moisture or hydrates or others, none of which are standard or normal, as they can be specific and peculiar to each individual source of the liquid CO.sub.2 or to a specific plant upset, any of which can in turn cause subsequent system or dispensing equipment malfunction.
An additional difficulty is that when utilizing industrial type, air cooled single stage, freon type mechanical refrigeration units for maintaining CO.sub.2 vessel temperatures/pressures much below 0.degree. F./300 psig; when lower temperatures/pressures are desired, the pressure ratio of the freon type compressor becomes excessive, and cooling capacity is lost. A currently-used unit loses nearly 35% of its capacity if attempting to reduce the R-404A evaporator temperature from 0.degree. F. to -20.degree. F. In addition, the compressor's performance becomes very sensitive to minor machinery problems.
Accordingly, it is evident that a number of systems have been devised for attempting to overcome some of these potential difficulties and deliver lower temperature CO.sub.2 liquid. Other examples of such systems include those shown in U.S. Pat. Nos. 4,100,759; 4,127,008; 4,137,723; 4,187,325; 4,211,085; and 5,177,974. Although some of these systems have worked satisfactorily for specific applications, none have solved all the problems, and consequently, an improved system has been sought. Among the difficulties to be solved are those occasioned by the different logistics of supply and use presented by these different users. Accordingly a system is needed that not only can totally control the temperature/pressure of the liquid CO.sub.2 in the storage vessel, but also be readily adaptable to a wide variation in use patterns and thus able to cope with these varied and diverse problems.
CO.sub.2 is also useful as a working refrigerant, especially when temperatures in the range of -65.degree. F. to -20.degree. F. are desired, because of its favorable pressure characteristics at those temperatures. Its use as the bottom stage refrigerant in a binary cascade system has been long recognized, but little used.